For #1: Yes, you can just hit the restore button in iTunes. All traces of a jailbreak will vanish.

For #2: There are conflicting reports. The reality is that Apple might be able to tell (though they'd never admit it, too many legal snooping issues), your carrier might be able to tell if you're using certain services that aren't on your account (tethering), but unless you're abusing the services, you probably don't have anything to worry about.

once jailbroken you will have access to Cydia, which is like the App Store but for jailbreakers. here you will find 100's of apps & tweaks that you can modify your device to your liking but unfortunately some, if not most, are not coded very well and in turn cause your device to not work very well. e.g. battery drain, imperfect UI transitions, apps not working properly and sometimes random springboard crashes

*you will find that the "better" apps & tweaks will cost you

as far as i know the carrier cannot tell and not sure if they really care.. unless you are tethering without their consent

Note: if you are really considering it i would suggest going down to the jailbreak section of this site and taking a long look around

Possibly, but if you can't get it to boot then chances are they can't either. If they can't, and it's still under warranty (or a store clerk's discretion period after the warranty expired, as was the case with my neighbor) then they'll just replace it no (or few) questions asked.

I was curious, so I asked my friend who is a brilliant IP Attorney (background in electrical engineering)

He said by law, Apple would need to service the device.

He said the technicality comes in when you load an unapproved version of software on the device, they can legally claim that they have no idea how to inspect, diagnose, or repair the device do to the unapproved software that was installed on it.

You can always un jail break by restoring or updating to new software in iTunes ... Unlocking is a diff story
Jailbreaking just gives you more control over your phone than apple let's you
Themes , tweaks etc change font etc give you option for stuff you didn't have standard .

Just for clarity to the op, if you jailbreak and have problems that you can't seem to fix even after asking all of us on here(which most likely some one will give you and awnser on how to fix said problem)and the phone is just plain not working(highly unlikely)then you just do a clean restore on iTunes and it will be as if the phone was brand new again. I suggest you try it and seen if its for you, if after you do so and feel you don't see what all the hype is about just restore and you're good to go. I would also suggest you save you're shsh blobs as soon as possible, this is important if you want to restore back to the firmware you were originally on. You can find very good jailbreak tutorials on here as well as learn how to save blobs with tiny umbrella. Good luck and have fun